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Scientists have discovered a menagerie of perfectly intact marine microorganisms trapped in tree resin at least 100 million years ago, according to a new study.

The unexpected find in the Charente region of southwestern France pushes back by at least 20 million years the period when a type of single-cell algae called diatoms are known to have appeared on earth, say the study's authors.

But the finding creates a mystery: how did sea creatures wind up trapped in a glob of resinated amber that oozes out of trees?

The most likely scenario, the scientists conclude, is that the forest producing the amber was very near the coast, and that the tiny organisms, which also included primitive plankton, were either carried inland by strong winds or flood waters during a storm.

"This discovery will deepen our understanding of these lost marine species as well as providing precious data about the coastal environment of western France during the Cretaceous Period," which spanned from 145 to 65 million years ago, say researchers.

It also challenges certain theories about the evolution of these organisms, and vindicates the research of molecular geneticists, says study co-author and National History Museum scientist Jean-Paul Saint Martin.

Using "molecular clocks," biochemists move backward in time to figure out at what point in the evolutionary process certain plant and animal species split off into different branches.

"We had no record of these microorganisms over a period of 20 million years. These fossils have filled that void in the most extraordinary manner," Saint Martin says.